(a) Field of the Invention
The present invention relates to a transflective liquid crystal display (LCD) device including a transmissive area and a reflective area in each pixel of the LCD device.
(b) Description of the Related Art
LCD devices are generally categorized in two types: a transmissive LCD device having therein a backlight unit as a light source; and a reflective LCD device having therein a reflection film which reflects external light incident onto the LCD device and thus functions as a light source. The reflective LCD device has the advantages of lower power dissipation, smaller thickness and lighter weight compared to the transmissive LCD device, due to absence of a backlight source in the reflective LCD device. On the other hand, the transmissive LCD device is superior to the reflective LCD device in that the transmissive LCD device can be well observed in a dark environment.
There is another type of the LCD device, known as a transflective LCD device, which has the advantages of both the reflective and transmissive LCD devices. Such a transflective LCD device is described in Patent Publication JP-A-2003-344837A, for example. The transflective LCD device includes a transmissive region (or transparent region), and a reflection region in each pixel of the LCD device. The transmissive region passes light emitted from a backlight source, and uses the backlight source as a light source. The reflective region includes a rear reflective plate or reflection film, and uses external light reflected by the reflection film as a light source.
In the transflective LCD device, the image display is performed by the reflective region in a well-lighted environment, with the backlight source being turned OFF, thereby achieving a smaller power dissipation. On the other hand, the image display is performed by the transmissive region in a dark environment, with the backlight source being turned ON, thereby achieving an effective image display in the dark environment.
In general, a variety of modes are used for operating LCD devices, including an in-plane-switching (IPS) mode, a twisted-nematic (TN) mode, and a fringe-field-switching (FFS) mode. Each pixel of the IPS-mode or FFS-mode LCD device includes a pixel electrode and a common electrode which are disposed on a common substrate to apply the liquid crystal (LC) layer with a lateral electric field. The IPS-mode or FFS-mode LCD device using a lateral electric field rotates the LC molecules in a plane parallel to the substrate to perform the image display, and achieves a higher viewing angle compared to the TN-mode LCD device.
If the IPS mode or FFS mode using a lateral electric field is to be employed in the transflective LCD device as described above, there arises an image-inversion problem in the LCD device, as described in the patent publication as mentioned above. More specifically, in a normal driving technique of the LCD device, if the transmissive region operates in a normally-black mode wherein absence of the applied voltage corresponds to a dark state, the reflective region operates in a normally-white mode wherein absence of the applied voltage corresponds to a bright state. The reason is of the image-inversion problem will be described in detail hereinafter.
FIG. 34A schematically shows a pixel of a transflective LCD device, which includes therein a reflective region 55 and a transmissive region 56. The transmissive region 56 is configured by a first polarizing film 51, a first substrate (counter substrate) 61, a LC layer 53 having a retardation of λ/2, a second substrate (TFT substrate) 62, and a second polarizing film 52, which are arranged in this order as viewed from the front of the LCD device 50, wherein λ is a wavelength of the light. The reflective region 55 is configured by the first polarizing film 51, first substrate 61, LC layer 53 having a retardation of λ/4, an insulation film 63, and a reflection film 54, as effective constituent elements. In FIG. 34A, polarizing axis of the polarizing films 51, 52, longer axis of the LC molecules in the LC layer 53 are depicted in the state wherein the LCD device is rotated by 90 degrees along a plane normal to the sheet of the drawing in the counterclockwise direction as viewed from the left of the drawing.
FIG. 34B shows polarization of light in the respective regions 55, 56 in FIG. 34A for the case of presence (Von) and absence (Voff) of the applied voltage, in the portions wherein the light passes through the first polarizing film 51, LC layer 53 and second polarizing film 52. In FIG. 34B, an arrow means linearly-polarized light, “L” encircled by a circle means counterclockwise-circularly-polarized light, “R” encircled by a circle means clockwise-circularly-polarized light, blank elongate bar means the director of the LC, i.e., longer axis of the LC molecules. FIG. 35 shows a sectional view of this type of the practical LCD device, the principle of which is shown in FIGS. 34A and 34B, including a backlight source 57.
In the LCD device 50a shown in FIG. 35, the reflective region 55 uses the reflection film 54 as the light source, whereas the transmissive region 56 uses the backlight source 57 as the light source.
The first polarizing film 51 disposed at the front side of the LC layer 53 and the second polarizing film 52 disposed at the rear side thereof have respective polarizing axes, which are perpendicular to one another. The LC layer 53 includes LC molecules having a director which is 90 degrees deviated from the polarizing axis of the second polarizing film 52 upon absence of the applied voltage. Assuming that the polarizing axis of the second polarizing film 52 is directed at a reference direction (zero degree), for example, the polarizing axis of the first polarizing film 51 is directed at 90 degrees and the longer axis of the LC molecules in the LC layer 53 is also directed at 90 degrees. The zero-degree direction is shown as the lateral direction in FIG. 34B, and the 90-degree direction is shown as the vertical direction in FIG. 34B. The cell gap of the LC layer 53 in the transmissive region 56 is adjusted such that the retardation Δnd is equal to λ/2, whereas the cell gap of the LC layer 53 in the reflective region 55 is adjusted such that the retardation Δnd is equal to λ/4, given λ, Δn and d being wavelength of the light, refractive-index anisotropy and cell gap, respectively. As for λ, if the wavelength of green light is used as a reference, λ is 550 nm.
Operation of the LCD device shown in FIGS. 34A, 34B and 35 will be described hereinafter, for each case of absence and presence of the applied voltage in respective regions 55, 56.
(1) Reflective Region Upon Absence of Applied Voltage:
In the left column (Voff) of the reflective region 55 shown in FIG. 34B, a linearly-polarized light polarized at 90 degrees, i.e., 90-degree linearly-polarized light is incident onto the LC layer 53 after passing through the first polarizing film 51. Since the optical axis of the linearly-polarized light incident onto the LC layer 53 is aligned with the longer axis of the LC molecules, the 90-degree linearly-polarized light passes through the LC layer 53 as it is, and is then reflected by the reflection film 54. The linearly-polarized light does not change the state thereof in general after the reflection, as shown in FIG. 34B, and is again incident onto the LC layer 53 as the 90-degree linearly-polarized light. The 90-degree linearly-polarized light passes through the LC layer 53 as it is, and is incident onto the first polarizing film 51, which has a polarizing axis at 90 degrees, passes the 90-degree linearly-polarized light as it is. Thus, absence of the applied voltage allows the reflective region to assume a bright state.
(2) Reflective Region Upon Presence of Applied Voltage:
In the right column (Von) of the reflective region. 56 in FIG. 34B, the 90-degree linearly-polarized light passed by the first polarizing film 51 is incident onto the LC layer 53. The voltage applied to the LC layer 53 directs the longer axis of the LC molecules from zero degree to 45 degrees within the plane parallel to the substrates. The deviation of polarized direction of the incident linearly-polarized light from the longer axis of the LC molecules in the LC layer 53 by 45 degrees and the retardation of λ/4 change the 90-degree linearly-polarized light into a clockwise-circularly-polarized light after the reflection, which is incident onto the reflection film 54 and reflected thereby. The reflected light shifts to a counterclockwise-circularly-polarized light and is incident onto the LC layer 53. The counterclockwise-linearly-polarized light is changed by the LC layer 53 into a zero-degree linearly-polarized light and incident onto the first polarizing film 51. The polarizing film 51 having a polarizing axis at 90 degrees blocks the incident light, thereby representing dark state.
Thus, the reflective region 55 operates in a normally-white mode wherein absence of the applied voltage provides a bright state, whereas presence of the applied voltage provides a dark state.
(3) Transmissive Region Upon Absence of Applied Voltage:
In the left column of the transmissive region 56 shown in FIG. 34B, a zero-degree linearly-polarized light is passed by the second polarizing film 52 and incident onto the LC layer 53. Since this incident light has a polarized direction normal to the longer axis of the LC molecules in the LC layer 53, the incident light is passed by the LC layer 53 as it is, and is incident onto the first polarizing film 51 as the zero-degree linearly-polarized light. The first polarizing film 51 having a polarizing axis at 90 degrees blocks the incident light, thereby representing a dark state.
(4) Transmissive Region Upon Presence of Applied Voltage:
In the right column of the transmissive region 56 shown in FIG. 34B, a zero-degree linearly-polarized light is passed by the second polarizing film 52 and incident onto the LC layer 53. The voltage applied to the LC layer 53 directs the longer axis of the LC molecules from zero degree to 45 degrees within the plane parallel to the substrates. The deviation of polarized direction of the incident linearly-polarized light from the longer axis of the LC molecules in the LC layer 53 by 45 degrees and the retardation of λ/2 of the LC layer change the zero-degree linearly-polarized light into a 90-degree linearly-polarized light, which is incident onto the first polarizing film 51. The first polarizing film 51 having a polarizing axis at 90 degrees passes the incident light, thereby representing a bright state.
Thus, the transmissive region operates in a normally-black mode wherein absence of the applied voltage provides a dark state whereas presence of the applied voltage provides a bright state.
The image-inversion problem is a general problem common to the lateral-electric-field modes (IPS mode, FFS mode) and other LCD modes. However, as to the TN mode, horizontal-orientation mode (ECB mode) or vertical-alignment mode (VA mode), for example, the image-inversion problem may be solved using a circularly-polarized light as the incident light to the LC layer. For this purpose, the orientations of the first polarizing film and λ/4 wavelength film are deviated by 45 degrees from one another. However, if the incident light is a circularly-polarized light, the circularly-polarized light looses the sensitivity to the rotation of the LC molecules parallel to the substrates, and thus passes through the LC layer as the circularly-polarized light. Accordingly, the LCD device using the lateral electric field represents a dark state at any time irrespective of presence or absence of the applied voltage in either of the reflective mode and the transmissive mode. That is, the lateral-electric-field-mode LCD device cannot represent the image thereof by using such a λ/4 wavelength film.
As described above, the transflective LCD device has the problem that both the absence and presence of the applied voltage provide reversed images of bright state and dark state in each pixel. The patent publication as mentioned above solves this problem without using the λ/4 wavelength film, by using the arrangement shown in FIG. 35, wherein the polarizing axis of the first polarizing film 51 is 45 degrees deviated from the longer axis of the LC molecules in the LC layer 53, as shown on the left side of the drawing. In this case, the reflective region 55 operates in a normally-black mode, whereas the transmissive region 56 operates in a normally-white mode. In order for changing the transmissive region 56 to operate in a normally-black mode, a λ/2 wavelength film 58 is interposed between the second polarizing film 52 and the LC layer 53, the λ/2 wavelength film 58 having an optical axis at 135 degrees, which is perpendicular to the longer axis of the LC molecules in the LC layer 53.
By using the above configuration, in the front viewing angle, the λ/2 wavelength film 58 compensates the polarizing effect on the light by the LC layer 53 having a retardation at λ/2. Thus, the combination of the LC layer 53 and λ/2 wavelength film 58 provides a substantially similar polarized state for both the incident light and the reflected light. Accordingly, the light passed by the second polarizing film 52 and assuming a 90-degree linearly-polarized state remains in the same polarized state after passing through the λ/2 wavelength film 58 and LC layer 53, and thus cannot pass through the first polarizing film 51. In short, the λ/2 wavelength film 58 interposed between the LC layer 53 and the second polarizing film 56 allows the transmissive region 56 to operate in a normally-white mode.
In the LCD device 50a shown in FIG. 35, the polarized direction of the light incident onto the LC layer 53 is deviated from the parallel or normal direction of the longer axis of the LC molecules in the LC layer 53. This involves a significant leakage of light during display of a dark state, due to the wavelength dispersion characteristic of the retardation of the LC layer 53. In addition, the λ/2 wavelength film 58 itself has a wavelength dispersion characteristic, which also causes leakage light during display to of a dark state.
It is to be noted that the image-inversion problem, wherein the transmissive region 56 and the reflective region operate in reverse normal modes, can be solved by inverting the polarity of the applied voltage between the transmissive region 56 and the reflective region 55. The inversion of the voltage polarity as used herein is such that absence of the applied voltage in the transmissive region 56 and presence of the applied voltage in the reflective region 55 are concurrently performed. However, this configuration is not known in the field of LCD devices. In addition, the problem encountered in such a configuration and the technique for solving the problem are also not known.